DESCRIPTION

ALTERTABLE changes the definition of an existing table. There are
several subforms:
ADDCOLUMN
This form adds a new column to the table using the same syntax
as CREATE TABLE [create_table(7)].
DROPCOLUMN
This form drops a column from a table. Indexes and table
constraints involving the column will be automatically dropped
as well. You will need to say CASCADE if anything outside the
table depends on the column, for example, foreign key references
or views.
ALTERCOLUMNTYPE
This form changes the type of a column of a table. Indexes and
simple table constraints involving the column will be
automatically converted to use the new column type by reparsing
the originally supplied expression. The optional USING clause
specifies how to compute the new column value from the old; if
omitted, the default conversion is the same as an assignment
cast from old data type to new. A USING clause must be provided
if there is no implicit or assignment cast from old to new type.
SET/DROPDEFAULT
These forms set or remove the default value for a column. The
default values only apply to subsequent INSERT commands; they do
not cause rows already in the table to change. Defaults may
also be created for views, in which case they are inserted into
INSERT statements on the view before the view’s ON INSERT rule
is applied.
SET/DROPNOTNULL
These forms change whether a column is marked to allow null
values or to reject null values. You can only use SET NOT NULL
when the column contains no null values.
SETSTATISTICS
This form sets the per-column statistics-gathering target for
subsequent ANALYZE [analyze(7)] operations. The target can be
set in the range 0 to 1000; alternatively, set it to -1 to
revert to using the system default statistics target
(default_statistics_target). For more information on the use of
statistics by the PostgreSQL query planner, refer to the
documentation.
SETSTORAGE
This form sets the storage mode for a column. This controls
whether this column is held inline or in a supplementary table,
and whether the data should be compressed or not. PLAIN must be
used for fixed-length values such as integer and is inline,
uncompressed. MAIN is for inline, compressible data. EXTERNAL is
for external, uncompressed data, and EXTENDED is for external,
compressed data. EXTENDED is the default for most data types
that support non-PLAIN storage. Use of EXTERNAL will make
substring operations on text and bytea columns faster, at the
penalty of increased storage space. Note that SET STORAGE
doesn’t itself change anything in the table, it just sets the
strategy to be pursued during future table updates. See the
documentation for more information.
ADDtable_constraint
This form adds a new constraint to a table using the same syntax
as CREATE TABLE [create_table(7)].
DROPCONSTRAINT
This form drops constraints on a table. Currently, constraints
on tables are not required to have unique names, so there may be
more than one constraint matching the specified name. All
matching constraints will be dropped.
CLUSTER
This form selects the default index for future CLUSTER
[cluster(7)] operations. It does not actually re-cluster the
table.
SETWITHOUTCLUSTER
This form removes the most recently used CLUSTER [cluster(7)]
index specification from the table. This affects future cluster
operations that don’t specify an index.
SETWITHOUTOIDS
This form removes the oid system column from the table. This is
exactly equivalent to DROP COLUMN oid RESTRICT, except that it
will not complain if there is already no oid column.
Note that there is no variant of ALTERTABLE that allows OIDs to
be restored to a table once they have been removed.
OWNER This form changes the owner of the table, index, sequence, or
view to the specified user.
SETTABLESPACE
This form changes the table’s tablespace to the specified
tablespace and moves the data file(s) associated with the table
to the new tablespace. Indexes on the table, if any, are not
moved; but they can be moved separately with additional SET
TABLESPACE commands. See also CREATE TABLESPACE
[create_tablespace(7)].
RENAME The RENAME forms change the name of a table (or an index,
sequence, or view) or the name of an individual column in a
table. There is no effect on the stored data.
All the actions except RENAME can be combined into a list of multiple
alterations to apply in parallel. For example, it is possible to add
several columns and/or alter the type of several columns in a single
command. This is particularly useful with large tables, since only one
pass over the table need be made.
You must own the table to use ALTERTABLE; except for ALTERTABLEOWNER, which may only be executed by a superuser.

PARAMETERS

name The name (possibly schema-qualified) of an existing table to
alter. If ONLY is specified, only that table is altered. If ONLY
is not specified, the table and all its descendant tables (if
any) are updated. * can be appended to the table name to
indicate that descendant tables are to be altered, but in the
current version, this is the default behavior. (In releases
before 7.1, ONLY was the default behavior. The default can be
altered by changing the configuration parameter
sql_inheritance.)
column Name of a new or existing column.
new_column
New name for an existing column.
new_name
New name for the table.
type Data type of the new column, or new data type for an existing
column.
table_constraint
New table constraint for the table.
constraint_name
Name of an existing constraint to drop.
CASCADE
Automatically drop objects that depend on the dropped column or
constraint (for example, views referencing the column).
RESTRICT
Refuse to drop the column or constraint if there are any
dependent objects. This is the default behavior.
index_name
The index name on which the table should be marked for
clustering.
new_owner
The user name of the new owner of the table.
tablespace_name
The tablespace name to which the table will be moved.

NOTES

The key word COLUMN is noise and can be omitted.
When a column is added with ADD COLUMN, all existing rows in the table
are initialized with the column’s default value (NULL if no DEFAULT
clause is specified).
Adding a column with a non-null default or changing the type of an
existing column will require the entire table to be rewritten. This may
take a significant amount of time for a large table; and it will
temporarily require double the disk space.
Adding a CHECK or NOT NULL constraint requires scanning the table to
verify that existing rows meet the constraint.
The main reason for providing the option to specify multiple changes in
a single ALTERTABLE is that multiple table scans or rewrites can
thereby be combined into a single pass over the table.
The DROP COLUMN form does not physically remove the column, but simply
makes it invisible to SQL operations. Subsequent insert and update
operations in the table will store a null value for the column. Thus,
dropping a column is quick but it will not immediately reduce the on-
disk size of your table, as the space occupied by the dropped column is
not reclaimed. The space will be reclaimed over time as existing rows
are updated.
The fact that ALTER TYPE requires rewriting the whole table is
sometimes an advantage, because the rewriting process eliminates any
dead space in the table. For example, to reclaim the space occupied by
a dropped column immediately, the fastest way is
ALTER TABLE table ALTER COLUMN anycol TYPE anytype;
where anycol is any remaining table column and anytype is the same type
that column already has. This results in no semantically-visible
change in the table, but the command forces rewriting, which gets rid
of no-longer-useful data.
The USING option of ALTER TYPE can actually specify any expression
involving the old values of the row; that is, it can refer to other
columns as well as the one being converted. This allows very general
conversions to be done with the ALTER TYPE syntax. Because of this
flexibility, the USING expression is not applied to the column’s
default value (if any); the result might not be a constant expression
as required for a default. This means that when there is no implicit
or assignment cast from old to new type, ALTER TYPE may fail to convert
the default even though a USING clause is supplied. In such cases, drop
the default with DROP DEFAULT, perform the ALTER TYPE, and then use SET
DEFAULT to add a suitable new default. Similar considerations apply to
indexes and constraints involving the column.
If a table has any descendant tables, it is not permitted to add,
rename, or change the type of a column in the parent table without
doing the same to the descendants. That is, ALTERTABLEONLY will be
rejected. This ensures that the descendants always have columns
matching the parent.
A recursive DROP COLUMN operation will remove a descendant table’s
column only if the descendant does not inherit that column from any
other parents and never had an independent definition of the column. A
nonrecursive DROP COLUMN (i.e., ALTERTABLEONLY...DROPCOLUMN) never
removes any descendant columns, but instead marks them as independently
defined rather than inherited.
Changing any part of a system catalog table is not permitted.
Refer to CREATE TABLE [create_table(7)] for a further description of
valid parameters. the documentation has further information on
inheritance.

COMPATIBILITY

The ADD, DROP, and SET DEFAULT forms conform with the SQL standard. The
other forms are PostgreSQL extensions of the SQL standard. Also, the
ability to specify more than one manipulation in a single ALTERTABLE
command is an extension.
ALTERTABLEDROPCOLUMN can be used to drop the only column of a table,
leaving a zero-column table. This is an extension of SQL, which
disallows zero-column tables.